In a Liquefied Petroleum Injection (LPI) system, Liquefied Petroleum Gas (LPG) fuel is generally directly injected into the vehicle engine to improve the engine output and reduce exhaust gas of an LPG vehicle. This conventional system however has a clear drawback, particularly when using a fuel pump driver, which is deactivated when its relay power is insulated by turning off an LPG switch. For instance, after turning off the LPG switch while the ignition is on, if an interface box due does not receive a signal from the fuel pump driver because the pump driver is deactivated, the signal line from the fuel pump driver is determined to be disconnected and an engine warning lamp erroneously illuminates. Though no signal is received from the fuel pump driver because the LPG switch is turned off, an Electronic Control Unit (ECU) of the interface box nevertheless determines the above state as a line-disconnection and illuminates the warning lamp.